The 'O' stands for 'overharvesting' and 'overhunting'

Many of the world's biggest and most impressive animals, such as whales, elephants, rhinos and tigers, are now all rare because they were hunted heavily in the past.

People wanted whale oil and whale meat, elephant ivory, and rhino and tiger trophies. Although all of these animals are now protected by law from excess hunting, illegal poaching still continues.

Other species are overharvested - they are used faster than they can be replaced - which is likely to lead to decline and extinction.

Cod is now too rare to be caught in many areas off the coast of America and in the North Sea - and the situation is the same for many other types of fish. Plant species can also be easily overharvested - the Brazil nut tree might be in danger of extinction, because not enough of its nuts are being left in the rainforest to grow into new trees.